Polyolefins currently find widespread use in many applications, from packaging to functional products, such as medical devices and disposable consumer products. They are safe, lightweight, and relatively inexpensive. Polyolefins are relatively easy to melt process into their final form via various forming operations, and they are also readily recycled due to their good thermal stability and inert characteristics. Because of these many characteristics, they are ubiquitous in modern society.
Commercially available polyolefins are currently derived from petroleum and/or natural gas, which are finite natural resources. Due to the finite nature of polyolefin feedstocks, the cost of polyolefin is linked to the price volatility of crude oil and natural gas. Economic, social, environmental, and political pressures to reduce dependence on finite resources, such as petroleum and natural gas, and to replace them with materials derived from renewable feedstocks have grown recently. Optimally, such renewable materials would have processability and performance characteristics, as well as cost structure, similar to those of traditional polyolefins. It is also desirable that such renewable materials retain the recylability of traditional polyolefins and do not significantly alter the recycling infrastructure in place today for high-density polyethylene (HDPE)
An example of a renewable thermoplastic material is polylactic acid (PLA), which is an aliphatic polyester derived from renewable agricultural products. PLA has been used in a number of applications, such as water bottles and packaging clamshells for fresh produce, to wholly replace polyolefin or polyethylene terephthalate (PET). PLA, however, is not in widespread use due to its limited processability (i.e., poor melt strength, which does not allow it to be extrusion-blow-molded into bottles), limited recyclability (i.e., lack of a dedicated recycling stream, potential to contaminate the PET recycling stream, as described below), and other disadvantageous properties of the material (i.e., low heat deflection temperature, poor water barrier, poor resistance to solvents and surfactants encountered in non-food packaging applications).
There have been efforts to modify some of the properties of PLA (i.e., poor melt strength) by adding other components to PLA, at minor concentrations (less than about 50 wt %). For example, polyolefins and their copolymers have been added to PLA at concentrations less than about 50 wt % to improve the impact properties of PLA. The processing of such PLA and polyolefin mixtures, though, unlike the processing of polyolefins, such as HDPE, requires additional pre-processing steps and the use of twin screw extruders as well as reactive melt strength enhancing additives (RMSEA), in order to achieve adequate dispersion of the minor polyolefin phase in the continuous PLA phase. Pre-processing requires that PLA pellets and polyolefin pellets are melted, mixed, cooled, solidified, and cut into PLA/polyolefin pellets, which are then fed into a twin screw extruder. The use of a twin screw extruder and/or a RMSEA adds significant cost and complexity to the manufacturing process. In extrusion blow molding, molders typically incorporate single screw extruders, not twin screw extruders. With regard to the RMSEA, the use of these additives requires monitoring and greater control of the manufacturing process. The use of a RMSEA may also require a purification step, to remove unreacted reactants, and it may produce volatile products, which necessitates the use of a twin screw extruder to vent such volatile products. Thus, the previously taught mixtures of PLA and polyolefin could not simply be substituted for a polyolefin, such as HDPE, and processed on a traditional polyolefin processing platform (i.e., a traditional extrusion blow molding platform), using a single screw extruder and without pre-processing or the use of a RMSEA.
Additionally, products made from such PLA and polyolefin mixtures are not currently recyclable, due to difficulties in separating such products from the primary PET recycling stream. There are two primary streams of plastic recycling, the HDPE stream and the PET stream (PET is a clear plastic and contamination with HDPE compromises its clarity, i.e., PET becomes hazy). Water-based separation systems are used to separate the two plastics by density, prior to recycling. The density of PET is greater than 1 g/cm3, the density of water, and it sinks in water-based separation systems. The density of HDPE is less than 1 g/cm3 and it floats in water-based separation systems. Containers made from a blend of HDPE and PLA, where PLA is present at a concentration of approximately 30% or greater, have a density greater than 1 g/cm3. Such containers sink in a water-based separation system, thereby contaminating the PET stream.
Therefore, there still exists a need for polymeric compositions that, in terms of processability, performance, recyclability, and cost, are similar to polyolefins but contain renewable polymers.